That was quite a mouthful of questions :-)

I will try to address some of them:

Torpen Monday, September 9, 2013

I'm a foreigner in Oslo so difficult for me to use friends to know about good companies. Is there a website somewhere I could use to search in the area of Oslo (with some marks or whatever) ?

Try one of these:
http://www.finn.no/oppdrag/ 
anbudstorget.no
mittanbud.no
mittoppdrag.no
riksanbud.no

Torpen Monday, September 9, 2013

My new project will require some permits from the kommune. How long will it take and how much details do I need to do that ? Do the companies take care of this or this is my responsability ? And how much can it cost in average?

Some things can be done without a permit, like tearing down an indoor wall that doesn't carry any load. 

Some things must be reported to the building authorities in the municipality.

Some jobs require an application.

Some types of  larger projects require an application from a company that has been granted the right to apply for a building permit. 

Your architect friend will know about these rules and will be able to help you find concrete information. 

Torpen Monday, September 9, 2013

I already changed a few things on my roof and I remember that the company doing the work was not able to give me an estimate for the whole work .... only hourly fee. But for a whole renovation, I guess, companies here in Norway should be able to give a full estimate, no ? That's mandatory for me so I hope that's something I can ask.

This is a relatively free country :-)  You are free to choose the company you want.  If that company insists on taking paid on a time&material basis and refuses to give you an offer with a fixed amount, then you are free to talk to somebody else. 

Torpen Monday, September 9, 2013

Do companies ask for some money in order to give a tillbud?

For projects of small or moderate size, that is not common here.   

Torpen Monday, September 9, 2013

What is in average the cost per square meter to do a renovation?

That depends extremely much on what needs to be done.

Norway is an expensive country. 

We have a strong tradition for DIY.  This can help significantly in keeping costs down. 

Torpen Monday, September 9, 2013

Does it cost a lot to move a fireplace (cheminée and fireplace)?

Having a new chimney built for you in a new house, without any difficulties or obstacles, will set you back around 20 to 30 grand.  In an old house there can be unpleasant surprises making this more complicated and thereby more expensive.  And then there is the cost of pulling down the old one, and fixing the roof or other building parts that might be affected.  And then there is the cost for the fire place... 

Moving a fireplace involves more work than just pushing it from one side of the room to another :-)

Be aware of the costs and hassle before you decide on this...

Torpen Monday, September 9, 2013

Is it better to buy myself my own material or let the company doing the job do it ?

For most types of materials, the difference is not dramatic.  Have a chat with with your chosen or candidate company about this. 

Torpen Monday, September 9, 2013

I want to do my whole garden as well ... any idea of cost per square meter in average ?

No, I have no idea about what your specific wishes will cost.  A full makeover of a tiny but complex backyard will cost more per square meeter than planting an apple tree in the middle of a huge field. 

Anyway, I wish you good luck!   

Hi KarstenBeate,

Thanks a lot for all your answers.
That helped.

I will try to find companies now !

Best regards,
Torpen

Torpen,
We're about half way through a major-ish renovation on an old house in stavanger (complete roof, and loft conversion, new garage etc). We spent quite a lot of time choosing a building contractor.
What we learnt was :

1) builders will offer fixed prices, if the Scope of work is well defined and there is not much uncertainty in the condition of the existing building. You need to have your architect to really define exactly what you want and document it properly in drawings and specifications to make this work. The contractor will price in risk though, so if there is a lot of uncertainy then an hours based contract might be better.

2) your architect should know the various builders you can use, but often architects also can be tied to a particular building company. Oslo may be different to stavanger, but I would expect there are a couple of big companies will provide the full service and a fixed price. Smaller companies will have a lower cost base and can be considerably cheaper...for our project we are using we had a huge range on the prices (though none offered fixed) with a small co coming in at  0.9million vs. 1.3million++....Customer care from a small company is a lot better too given experiences from some of my friends.

3) Perhaps the most important thing is to ask for references, and then go and see recent jobs similar to yours and talk to the owners to see if they are happy with what they got.

4) unless the work is very simple, avoid polish (etc) companies, and make sure that you use a real company with insurance, registrations (ask about which tiltaks klasse they have), and experienced workers (ensure the foreman has experience in what you are planning). Even if the cost they offer is much less avoid unregistered black work. We've used foreign companies and this can work but only when it's simple stuff. Also if you need work done on a roof or whatever you' ll want a proper company as an arbeidstilsyn (work inspector)  visit with an unqualified contractor is not much fun, and also you'll want guarantees you'll be covered.

5) if you are going to change the outside of the house (facade endring) at all, or roof, then talk to the local komune building department, and if you've already got some drawings, ask for a forhandskonferanse - with the planning saksbehandler. They will tell you whether you should apply and then whether you are likely to get a yes from the komune.

Hope this is helpful!